Electroless deposition of cobalt boron

ABSTRACT

Improved electroless deposits of cobalt-boron which find particular use in protecting basis metals against corrosive attack, the deposits being prepared from an acid bath having the following preferred composition:   D R A W I N G

United States Patent 1 Pearlstein et al.

[ Nov. 4, 1975 ELECTROLESS DEPOSITION OF COBALT BORON [75] Inventors: Fred Pearlstein; Robert F.

,Weightman, both of Philadelphia,

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

[22] Filed: July 20, 1973 [21] Appl. No.: 381,121

[52] US. Cl. 29/l96.6; 148/62; 427/438; 427/405; 427/ 131 [51] Int. Cl. C23B 3/00 [58] FieldofSearch 117/130 E, 130 8,71 M; 148/62, 6.21; 106/1; 29/1966; 204/38 B, 38

[56] References Cited UNITED STATES PATENTS 5/1959 Duvall 117/130 E X 3/1961 De Long et al.... 4/1966 Klein et a1. 148/62 X Primary Examiner-Ralph S. Kendall Assistant Examiner-Charles R. Wolfe, Jr. Attorney, Agent, or Firm-Arthur M. Suga; Nathan Edelberg; Robert P. Gibson 57 ABSTRACT I Improved electroless deposits of cobalt-boron which find particular use in protecting basis metals against corrosive attack, the deposits being prepared from an acid bath having the following preferred composition:

Cobalt sulfateJHiO Sodium succinatebH O Sodium sulfate Dimethylamine botane 2*Claims, No Drawings ELECTROLESS DEPOSITION OF COBALT BORON The invention described herein may be -manufactured, used and licensed by or for the Government for governmental purposes without the payment to. us of any royalty thereon. v

This inventionrelates to electroless deposition and more particularly concerns improved bath compositions for the electroless deposition of cobaltboron.

An object of the present invention is to provide an improved acid bath solution yielding an electroless cobalt-boron alloy deposit which is sound, decorative, and free of pits and cracks.

Another object of the invention is to provide such an alloy deposit which is particularly useful in protecting basis metals against corrosive attack.

Still another object of the invention is to provide such an alloy deposit which may readily be chromate treated to provide exceptional resistance to tarnishing.

Yet another object of the invention is to provide such an alloy deposit which is useful for providing magnetic deposits of low coercive force.

These and other objects of the invention will become apparent as the invention is further described hereinafter.

We have discovered an improved acid electroless cobalt-boron plating bath consisting of cobalt sulfate, sodium succinate, dimethylamine borane (hereinafter referred to as DMAB), and sodium sulfate. The succinate prevents formation of highly stressed, cracked deposits, which characterized prior art cobalt-boron deposits made from acetate-containing solutions. Even with the improved cobalt-boron deposits provided by succinate baths, however, pitted deposits resulted therefrom which offered only minimal resistance to corrosive attack of the basis or substrate metal. We have discovered that sodium sulfate very substantially reduces pit- -ting in cobalt-boron alloy deposits.

More specifically, improved bath compositions de- DMAB 4 The bath will normally be used at a pH of about 5.0

and about 70C.

Electroless cobalt-boron deposition rates were only slightly affected by cobalt concentration within the range of to 45 g/l; reduced deposition rates resulted beyond those limits.

DMAB concentration on deposition rate was determined from solutions containing g/l cobalt sulfate.7- H O and 15 g/l sodium succinate.6H O. The baths were adjusted to pH 5.0 with H 80 solution and were operated at 80C. Deposition rates increased essentially linearly with concentration to a maximum of about 4g/l. Above, this concentration, catalytic particles of cobalt formed in the solution which settled and resulted in rapid bath decomposition.

The effect of succinate concentration on deposition 80C. When no succinate was present, spongy cobalt formed within the solution accompanied by rapid bath decomposition and virtually no deposition was produced 'on palladium-activated copper test specimens. Satisfactory bath stability however was provided by the presence of lO g/l, up to about 30 g/l, of the succinate in the bath. The succinate also provided buffering action, i.e., prevented rapid change in bath pH during deposition. Deposition rates were highest and essentially constant at succinate hexahydrate concentrations of about 15 to 30 g/l, with about 2.5 g/l appearing to yield the best deposition characteristics.

15 g/] or more of Na SO provided effective anti-pitting characteristics to the deposits. At Na SO concentration of 10 g/l or less, pitting could be observed in the final cobalt-boron alloy deposit.

The effect of bath pH on deposition rate at 80C was determined. Deposition rate increased markedly with an increase in bath pH, but above 5.5, baths tended to be unstable and deposits formed on the walls and the bottom of the beaker.

It is interesting to note that baths at pH 4.5 and lower, increased in pH during the deposition tests, while baths at pH 5.5 and above, decreased. Normally, electroless plating baths decrease in pH during use because hydrogen ions will be formed as a product of the electroless reaction. DMAB, however, is subject to acid catalyzed hydrolysis: I (CH' HNBH +3I-l O-l-H C H NH +H BO +3H Indeed, we have found that even when no electroless plating was occurring, baths (80C) at pH 4.5 and below increased in pH with passage of time and that gassing was evident within the solutions. At bath pH of 5.0, our baths were very stable, and little or no change in pH resulted during deposition. Unless otherwise noted, all bath compositions hereinafter used and tested had a pH of 5.0.

The rate of cobalt-boron deposition increased with increasing bath temperature in a manner typical of most electroless plating baths. At 90C or above, the bath was subject to spontaneous production of cobaltboron particles and rapid decomposition. At 80C, the bath was stable during one hour plating tests. A tendency for catalytic particle formation in the bath was observed during more extended plating periods at 80C causing bath decomposition. The bath will be less susceptible todecomposition if trace amounts of a catalytic poison such as l to 2 mg/l of lead acetate trihydrate, or thiorea, are introduced therein. However, at bath temperatures of 80C and above, consumption of DMAB by hydrolysis is significantly wasteful and it is thus advisable, from a practical standpoint, to operate the bath at lower temperatures unless very high deposition rates are needed. At 70C, the bath is operable without the necessity for addition of catalytic poison stabilizers and hydrolysis losses are minimized.

Deposition ratesare comparatively low at bath temperatures of 40 C and lower. The baths are usable even at room temperatures of 22 to 27C for applying thin, electrically conductive deposits to palladium-activated nonconductors, provided the bath pH is increased to I about 6.3. u-

spontaneously initiate deposition by immersion in our preferred acid electroless cobalt-boron plating bath, at

v pH 5.0 and C, are steel, electrolessn'ickel, palladium rate was determined from solutions containing 25 g/l i cobalt sulfate.7H O and 4 g/l DMAB'at pH 5.0"and placementdeposit of cobalt initially formed on the and gold. Alurr'iinum is spontaneously plated by a dis- 3 metal during immersion in the bath.

Copper, brass, silver, platinum, titanium, stainless steels and nonconductors usually do not initiate electroless cobalt-boron deposition unless one of the following steps is taken:

a. activation of the surfaces by nucleation with a catalytically active metal such as palladium, for example.

b. contacting the metal in the electroless plating bath with an actively plating metal which initiates deposition by galvanic action.

c. momentary application of sufficient cathodic current to the metal in the electroless plating bath to apply a thin cobalt-boron film electrolytically, thus initiating deposition by electrolytic action.

Our electroless cobalt-boron alloy deposit, when made from our acid bath, pH 5.0, 70C, of preferred concentrations, has the following composition, by weight:

Co 96.0% B 1.7% C 97% N 05% Carbon and nitrogen indicate the presence of organic or organometallic compounds in the deposit.

Our electroless cobalt-boron deposits possessed a hardness of 270 kg/mm (Vickers) in the as-plated condition. After heating the deposit however, for 24 hours at 250C, hardness of the deposit increased to 480 kglmm Additional heating at 250C further increased hardness to a maximum of about 640 kg/mm after a total heating period of 4 days.

The coercive force of our electroless cobalt-boron deposits was generally constant, at about 8 oersteds, over the range of thicknesses tested, i.e., at 17500 to 47100 Angstroms.

The salt spray corrosion resistance of steel plated with our deposits is compared to that of electroless nickel-phosphorus plated steel, also highly resistant to salt spray, and, is presented in Table II below:

TABLE ll Salt Spray Exposure Test 4 Edge corrosion of the substrate was very pronounced with the electroless nickel-phosphorus plated steel whereas the electroless cobalt-boron deposits provided good resistance to substrate edge corrosion. Good edge corrosion resistance of substrate metals is vital when used in sensitive mechanisms such as fuses and other military items where any dislodging of corrosive products could readily cause serious malfunctioning. Secondarily, discoloration due to edge corrosion, incipient or advanced, might be considered aesthetically unappealing.

The corrosion potential of electroless cobalt-boron deposits, immersed in g/l NaCl solution for 24 hours is -0.60 volts versus the saturated calomel electrode (S.C.E.). In the same solution, steel has a corrosion potential of 0.63 volts (S.C.E.) and electroless nickelphosphorus has a corrosion potential of 0.35 volts (S.C.E.). Our cobalt-boron deposit, with a corrosion potential of 0.60 volts (S.C.E.) is substantially more compatible electrochemically with steel than the electroless nickel deposit having a corrosion potential of 0.35 volts (S.C.E.). Parenthetically, a deposit having a corrosion potential identical with the steel under these conditions would not accelerate corrosion of the steel substrate. Thus, our cobalt-boron deposit, having a more favorable corrosion potential than electroless nickel deposits, will tend to better resist corrosion of basis metal exposed at any coating defect or pore by adverse galvanic cell action.

A double-layer deposit of electroless nickel-phosphorus followed by a coating of electroless cobaltboron will provide synergistic protection to the basis metal because of the ability of the cobalt to sacrificially protect the nickel layer from corrosive penetration. Double-layer deposits, i.e., 7.5 microns of electroless nickel and 2.5 microns of electroless cobalt-boron, on a substrate of steel were found capable of preventing basis metal attack (even at edges) after 168 hours salt spray exposure and were superior to the same total thickness of either layer alone.

Electroless cobalt-boron deposits tarnish rapidly during salt spray exposure to produce a tenacious, mottled Corrosion Rating* Deposlt After Exposure to Salt Spray Deposit Thickness, Microns 24hrs 48hrs 7 2hrs 96hrs l68hrs Electroless Co-B 3 5 4 3+ 3(sl.E) 2(5) 5 '5 5 4 3+ 3(sl.E)

l0 5 5 5 5 3-H sl. E) Electroless NH 3 4(E) 4(E) 4 E) 3+(E) -HE) (from acid-type bath) 6 5(E) 5(E) 5( 12 5 5(E) 5(E) 5(E) 5(E) Rating: 5 no basis metal attack 4 traces of basis metal attack 3 slight basis metal attack 2 moderate basis metal attack I considerable basis metal attack E edge corrosion sl. E slight edge corrosion All results are expressed as an average of 3 tested blue-brown film. Some success was achieved in providspecimens. Salt spray exposure test is described in ASTM Designation B-l l7-64, 31 August 1964, Standard Method of Salt Spray Testing.

As is apparent from Table ll, each of the electroless cobalt-boron or nickel-phosphorus deposits were protective to steel for 48 hours salt spray exposure. With longer exposure times, the protective value of the coatings tended to improve as coating thickness increased.

ing a black non-reflective surface so often desired in military equipment by immersing the deposit for 10- minutes in a 10 g/l potassium persulfate solution at 25C.

Immersion of the cobalt-boron deposit in a solution consisting of 200 g/l Na Cr O .2H O and 6 ml/l H sp. gr. 1.84, greatly improved the resistance of the deposit surfaces to tarnishing, the surfaces remaining.

bright and untamished after 72 hours salt spray exposure. The above solution however, is mildly corrosive to the cobaltboron deposit and will remove about 0.1 micron of deposit thickness during a second immersion. No visible film is produced by this treatment.

In the production of our improved cobalt-boron alloy deposit, the following steps should be observed:

a. Remove any organic contaminants such as oils, greases, etc. from the metal substrate surfaces by soak-alkaline cleaning, or if heavy contamination exists, a vapor degrease prior to soak-alkaline cleaning may be desirable. Electro-cleaning operations may also be applied.

b. Oxide or corrosion products will now be removed from the substrate surfaces, such as by immersion in a 50% (vol) l-lCl solution. A 10% (vol) H 50 dip, or a chemical polishing operation, is satisfactory for copper or copper alloy substrates. Aluminum may be immersed in 50% -(vol) HNO for removal of oxides and a thin zinc immersion deposit on aluminum by treatment in zincate solution is desirable for rapid, uniform coverage with cobalt.

c. The cleaned substrate metal may now be directly immersed in our solution, or if the metal does not initiate electroless cobalt-boron deposition, then one of the steps outlined above for this class of substrates should be followed. Deposit thickness will depend, among other things, upon the amount of time the metal is permitted to remain in the bath.

There is set forth hereinbelow for purposes of illustration, examples of our cobalt-boron deposits prepared under varying conditions:

Thickness of deposit 13 microns (about 0.5 mil) EXAMPLE II Bath composition Preferred (1 mg/l lead acetate added) Substrate Copper Bath temperature C Bath pH 5.5 Deposition time l hour Thickness of deposit 25 microns (about 1.0 mil) Preferred Palladium-activated glass (glass initially abrasive blasted for better deposit adhesion) 30 min.

0.3 micron (approx) Bath composition Substrate Bath temperature Bath pH Deposition time Thickness of deposit We wish it :to be understood that we do not 'desire to be limited to the exact details described for obvious modifications will occur to a person skilled in the art.

We claim:

1. A double layer deposit on steel for providing synergistic protection to said steel against corrosive attack in a saline environment, said double layer deposit comprising an initial electroless nickel-phosphorus layer and a coating thereover of electroless cobalt comprising by weight, 960% cobalt, 1.7% boron, 0.97% carbon, and 0.05% nitrogen.

2. Double layer deposit as described in claim 1 further characterized by said nickel-phosphorous layer being about 7.5'microns in thickness and said electroless cobalt layer being about 2.5 microns in thickness. 

1. A DOUBLE LAYER DEPOSIT ON STEEL FOR PROVIDING SYNERGISTIC PROTECTION TO SAID STEEL AGAINST CORROSIVE ATTACK IN A SALINE ENVIRONMENT, SAID DOUBLE LAYER DEPOSIT COMPRISING AN INITIAL ELECTROLESS NICKEL-PHISPHOROUS LAYER AND A COATING THEREOVER OF ELECTROLESS COBALT COMPRISING BY WIGHT, 96.0% COBALT, 1.7% BORON. 0.9% CARBON, AND 0.05% NITROGEN.
 2. DOUBLE LAYER DEPOSIT AS DESCRIBED IN CLAIM 1 FURTHER CHARACTERIZED BY SAID NICKEL-PHOSPHOROUS LAYER BEING ABOUT 7.5 MICRONS IN THICKNESS AND SAID ELECTROLESS COBALT LAYER BEING ABOUT 2.5 MICRONS IN THICKNESS. 